Problem: Which of these numbers is prime? ${21,\ 29,\ 69,\ 75,\ 98}$
A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 29 are 1 and 29. The factors of 69 are 1, 3, 23, and 69. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 98 are 1, 2, 7, 14, 49, and 98. Thus, 29 is a prime number.